Add Walkthroughs

[gnikit]

VS Code supports walkthroughs to make setting up and getting started with an extension easier see: https://code.visualstudio.com/api/references/contribution-points#contributes.walkthroughs

The walkthrough(s) should include:

• Installing a fortran compiler, gfortran or intel
• Setting up the linter
• Installing and setting up the Language Server
• Compiling a sample code
• Debugging a sample code